Answer
$-2x+y=0$
Work Step by Step
The result of exercise 32 tells us that
${\bf v}=\langle a,\ b \rangle$ is parallel to the lines $bx-ay=c$
Here, ${\bf v}=\langle -1,\ -2\rangle$ and the lines are $-2x+y=c.$
Since $P(1,2)$ is on the line, its coordinates satisfy the line equation
$\left\{\begin{array}{l}
-2(1)+(2)=c\\
c=0
\end{array}\right.$
So the line equation is: $\quad -2x+y=0$
To graph the line, use the given point and $(0,0).$
